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Chapter 12 Meiosis

Homologous chromosomes carry the same genes but each

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Homologous chromosomes carry the same genes, but each homolog may contain different alleles. The Concept of Ploidy Karyotype is the number and types of chromosomes present in an organism. Humans, oak trees, and other organisms that have two versions of each type of chromosome are called diploid. Diploid organisms have two alleles of each gene- one on each of the homologous pairs of chromosomes. Organisms whose cells contain just one of each type of chromosome- bacteria, algae, and archaea- are haploid. Haploid organisms don’t contain homologous chromosomes. They have just one allele of each gene. Biologists use a compact notation to indicate the number of chromosomes and chromosome sets in a particular organism or type of cell: o The letter n stands for the number of distinct types of chromosomes in a given cell and is called the haploid number. If sex chromosomes are present, they are counted as a single type in the haploid number. In humans, n is 23. o To indicate the number of complete chromosome sets observed, a number is placed before the n . Thus, a cell can be n , 2n , or 3n , etc. Human autosomal cells are 2n . The combination of the number of sets and n is termed the cell's ploidy. Diploid cells or species are designated 2n , because two chromosomes of each type are present- one from each parent. A maternal chromosome comes from the mother; a paternal chromosome comes from the father. Humans are diploid; 2n is 46. Haploid cells are labeled simply, n , because they have just on set of chromosomes- no homologs are observed. Stating a cell's ploidy is the same as stating the number of haploid chromosome sets present. It is common for species in some lineages- particularly plants- to contain more than two of each type of chromosome. Polyploid species may have 3 or more of each type of chromosome in each cell. Depending on the number of homologs present, polyploid species are called triploid ( 3n ), tetraploid ( 4n ), hexaploid ( 6n ), octaploid (8n ), etc. An Overview of Meiosis At the start of meiosis, chromosomes are in the same state they are in prior to mitosis.
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When chromosome replication is complete, each chromosome consists of two identical sister chromatids. Sister chromatids contain the same genetic information. They are physically joined at a portion of the chromosome called the centromere as well as along their entire length. Relationship between chromosomes and sister chromatids: o An unreplicated chromosome consists of a single DNA molecule with its associated proteins while a replicated chromosome consists of two sister chromatids. An unreplicated chromosome is a single thread; a replicated chromosome has paired threads. o Unreplicated and replicated chromosomes are both considered single chromosomes- even though the replicated chromosome comprises of two sister chromatids. Meiosis consists of two cell divisions, called meiosis I and meiosis II.
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Homologous chromosomes carry the same genes but each...

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